For an encapsulant of a semiconductor device, an esterified substance is a useful additive for improving continuous moldability and reflow resistance. Such an esterified substance can be synthesized from a polymer of an acid anhydride and 1-alkene, and an alcohol (Patent Document 1).
Since a reaction, in which a polymer of an acid anhydride and a 1-alkene and an alcohol are reacted in order to obtain an esterified substance, is slow, a catalyst is required. In Patent Document 1, p-toluenesulfonate is used as a catalyst in order to cause the reaction. However, there are cases in which sulfonic acid remains in an obtained reaction product. In a case in which an esterified substance in which such p-toluenesulfonate remains is used as an encapsulant of a semiconductor device, there are cases in which this can lead to poor insulation of the semiconductor device. Therefore, in order to use the esterified substance obtained using the method of Patent Document 1 as an encapsulant of a semiconductor device, it is necessary to reduce residual sulfonic acids to a level at which defects such as poor insulation of the semiconductor device do not occur. Sulfonic acids can be removed by washing the reaction product, but removal through washing is troublesome in terms of operation, and the removal efficiency of sulfonic acids is not necessarily high. In addition, in a case in which a reaction product is excessively washed, there are cases in which a low-molecular-weight esterified substance or a 1-alkene which contributes to the reflow resistance and mold release properties that are necessary for encapsulating of a semiconductor device are removed.